arXiv subjects

Lixia Yuan

Publications and source records attributed to Lixia Yuan.

At least 19 recordsLinked to original sources

arXiv2026

Dimension-Dependent Asymptotic Dynamics for Mean Curvature Flow with Robin Boundary Conditions

We consider a graphical mean curvature flow in a cylinder with Robin boundary conditions, which arises as a geometric model for interface motion in the singular limit of the Allen--Cahn equation with nonlinear boundary conditions. It was shown in [26] that, in the planar case, every solution converges to a translating Grim Reaper with a \emph{fixed profile} and \emph{finite speed}. In this paper, we investigate the radially symmetric problem in higher dimensions and reveal a completely different asymptotic dynamics caused by the spatial dimension. In contrast to the planar case, there is no fixed translating profile governing the long-time behaviour. Instead, the solution propagates with an exponentially increasing speed, while both the gradient $|Du|$ (away from the center) and the instantaneous speed $u_t$ diverge exponentially as $t\to\infty$. This reveals a fundamentally different asymptotic behaviour induced by the interaction between the Robin boundary condition and the spatial dimension, that is, the translating profile continuously degenerates and becomes asymptotically ray-like. Since the equation becomes asymptotically degenerate and no uniform-in-time $C^0$, $C^1$, or $C^2$ estimates are available, our analysis relies on a new approach based on the zero number argument.

math.DG↗

arXiv2026

CO Structures with Narrow Lines in Nearby Quiescent Regions

Using CO data from Phase I of the Milky Way Imaging Scroll Painting (MWISP) survey, we present a systematic study of molecular structures with narrow lines. We identify 57 CO structures, most of which exhibit low densities and subsonic/transonic turbulence. Among them, structures with large projected areas and diffuse, sheet-like geometries are identified as veil clouds. The low LSR velocities and the concentration of these CO structures toward both the Galactic center (e.g., Ophiuchus, Aquila) and anticenter (e.g., Cepheus, Taurus) regions suggest a local origin for the sample, as supported by distance measurements of about 200--300pc for a subset with relatively large angular extents. These nearby structures likely arise from large-scale compression driven by past supernova activity within the Local Bubble. The observed low-velocity-dispersion emission may trace quiescent regions where turbulence has decayed due to a lack of sustained energy injection. For diffuse veil clouds with an assumed magnetic field of ~10uG, ion-neutral friction may provide an additional mechanism for turbulent dissipation on sub-parsec scales corresponding to their thickness of 0.1--0.3pc. Tracing the atomic-to-molecular transition, veil clouds provide a unique window into the diffuse, quiescent precursor state of dense gas. They likely represent a widespread but previously overlooked component of the Galactic molecular gas reservoir, with significant implications for cloud formation and evolution, the total mass budget and spatial distribution of molecular gas, and the initial conditions of star formation as a related consequence.

astro-ph.GA↗

arXiv2026

Statistical Properties of Molecular Clouds in the Milky Way: Insights from Three-Isotopologue CO Observations of the MWISP Project

We present a comprehensive statistical analysis of molecular cloud (MC) properties using the MWISP survey's 12CO, 13CO, and C18O (J = 1--0) data toward the inner (l = 45$^\circ$--60$^\circ$) and outer (l = 120$^\circ$--130$^\circ$) Galaxy. From a strict selection of 24,724 identified MCs, a final sample of 3,161 well-resolved MCs is established. We investigate the distributions of observational, morphological, and derived physical parameters, as well as their environmental dependencies and intercorrelations. Our analysis reveals that MCs are typically oblate and tend to align with the Galactic disk. A critical evaluation using a nearby subsample confirms significant distance-dependent selection effects for some parameters, nevertheless, the direction of changes in these parameters can indicate distance influence. We also examine several specific subsamples, revealing the distinct characteristics of MCs in the G120 spiral shock region, MCs in the G50 interarm spurs, C18O-bright MCs, and MCs with supra-Larson velocity dispersion. For instance, MCs with supra-Larson velocity dispersion are predominantly small and likely young clouds inheriting turbulence from the diffuse ISM. Notably, a comparison across tracers reveals that typical MCs have a turbulent, diffuse, 12CO-bright gas structure in their outer layers that does not contribute directly to star formation. In contrast, 13CO-bright gas represents a turning point where gravity becomes significant; C18O-bright gas is about gravity-dominated. Comprehensive correlation analysis confirms a flatter $σ_v$-size relation than classic Larson's law and a strong mass-size relation. Incorporating dimensional analysis, we derive minimal sets of eigenparameters from which most other observational and physical parameters can be estimated. This highlights the underlying scaling relations that governing cloud properties.

astro-ph.GA↗

arXiv2026

PMO Polaris CO survey. II. Where is the dust?

Dust plays critical chemical and dynamical roles in the interstellar medium (ISM), but its specific association with molecular and atomic gas remains difficult to isolate. Combining the PMO Polaris CO Survey (PPCOS), EBHIS \ion{H}{I} data, and \textit{Planck} dust maps, this study investigates dust distributions across multiple gas components in the Polaris Flare. We employ multi-technique linear decomposition -- including full-spectrum fitting and a regularization approach -- to reconstruct the dust distribution from multi-component gas emissions. This framework quantifies dust contributions from CO-associated, \ion{H}{I}-associated, and CO-dark molecular gas phases. CO-associated dust accounts for 20--40\% of the total dust mass, whereas dust in the broad \ion{H}{I} (warm neutral medium, WNM) component is negligible. Instead, \ion{H}{I}-associated dust concentrates primarily within the narrow cold neutral medium (CNM) and a distinct, ultra-narrow component with a velocity width comparable to the \ion{H}{I} spectral resolution. Residual dust at atomic-to-molecular (\ion{H}{I}--CO) interfaces contributes 4--10\% to the global dust mass, but exceeds 25\% at molecular cloud boundaries, confirming a substantial presence of CO-dark molecular gas. Furthermore, the velocity fields of dust-associated \ion{H}{I} closely match those of CO, indicating active dynamical coupling between CO-emitting gas and the surrounding CNM. Guided by these results, we present a stepwise schematic cartoon illustrating the coupling between multi-phase gas structures, molecular formation, and dust growth.

astro-ph.GA↗

arXiv2026

PMO Polaris CO survey. I. A 100 deg$^2$ view of the Polaris Flare

Large-area CO surveys are essential for studying molecular cloud dynamics and evolution; however, most have focused on the Galactic plane, leaving high-latitude clouds less explored. We present the PMO Polaris CO Survey (PPCOS), which maps a 100~deg$^2$ region of the Polaris Flare in the $J=1-0$ transitions of $^{12}$CO, $^{13}$CO, and C$^{18}$O using the Delingha 13.7~m telescope. As the first large-area CO survey at high Galactic latitude ($|b| > 20^{\circ}$) with sub-arcminute resolution, PPCOS achieves sensitivities of $\sim$0.46~K for $^{12}$CO and $\sim$0.23~K for $^{13}$CO and C$^{18}$O at a spectral resolution of 0.16~km~s$^{-1}$ and an angular resolution of 50\arcsec. The $^{12}$CO emission reveals seven distinct complexes, where only $\sim$10\% of pixels display multiple velocity components, alongside a global velocity gradient of 0.18~km~s$^{-1}$~pc$^{-1}$. Typical line widths are $1.2 \pm 0.6$~\mbox{km~s$^{-1}$} for $^{12}$CO, while $^{13}$CO components are systematically narrower ($\lesssim 0.7\,ΔV_{\rm ^{12}CO}$). The $^{12}$CO/$^{13}$CO intensity ratios (5--25) indicate widespread $^{12}$CO optical thickness, resembling conditions found in giant molecular clouds (GMCs). Globally, the CO emission divides into two groups: a major group aligned with the velocity gradient and a secondary group elongated perpendicular to it, possibly regulated by large-scale coherent dynamics. We propose a three-layer hierarchy: a dynamically assembling and dispersing periphery traced by $^{12}$CO, a more stable intermediate kernel traced by $^{13}$CO, and gravitationally bound compact cores traced by C$^{18}$O. No young stellar objects are firmly associated with the molecular gas. PPCOS provides an ideal laboratory for studying turbulence, hierarchical structure, and early cloud evolution in a nearby, relatively simple molecular cloud.

astro-ph.GA↗

arXiv2026

A study of the Physical Properties and Star Formation Activity of a Large Sample of Molecular Clouds: I Distances

Accurate distances to molecular clouds are crucial for determining their physical properties, understanding star formation, and tracing Galactic spiral structure. A number of 103,517 molecular clouds has been identified by the DBSCAN algorithm in the MWISP Phase I CO survey (l = 9.75-229.75 deg, |b| <= 5.25 deg), most of which lack reliable distances. In this work, we propose three independent methods, all of which match the molecular cloud's velocity-integrated intensity maps of 12CO lines from the MWISP with the three-dimensional dust extinction maps derived from Gaia, Pan-STARRS 1, and 2MASS, to determine molecular cloud distances. We present a catalog of 1,573 molecular clouds with robust distances ranging from approximately 150 pc to 3000 pc, 90 percent of which are measured for the first time, with typical statistical and systematic uncertainties of approximately 20% and 10%, respectively. We also derive their physical properties, such as their mass and sizes. This publicly available catalog of molecular clouds with distances provides a foundation for testing molecular cloud scaling relations and probing how cloud conditions influence star formation across diverse Galactic environments.

astro-ph.GA↗

arXiv2026

The radial velocity stability for reference sources in the MWISP CO survey

To assess the velocity stability of CO spectral lines in the Milky Way Imaging Scroll Painting (MWISP) survey, we employ a cross-correlation method to measure velocity shifts across $\sim$ 10,000 CO spectra from six reference sources observed over the ten-year duration of the survey. The standard deviations ($σ$) of these measured velocity shifts range from 0.03 to 0.23 km s$^{-1}$ for their $^{12}$CO lines and 0.03 to 0.16 km s$^{-1}$ for $^{13}$CO lines. We find that larger shifts are associated with broader linewidths, more pronounced differences between monthly and long-term variations, and a stronger correlation between velocity shifts of $^{12}$CO and $^{13}$CO lines. By examining the relation of velocity shifts with the Azimuth-Elevation of the telescope, as well as the velocity fields of these extended sources, we find that the velocity shifts exhibit systematic changes across different Azimuth-Elevation ranges. The patterns and amplitudes of these changes vary among sources and are closely linked to the extended velocity fields of sources. This indicates that the increased velocity shifts are primarily caused by pointing errors, which are more sensitive to reference sources with higher velocity gradients. We also provide high signal-to-noise, velocity-aligned template spectra for reference sources.

astro-ph.GA↗

arXiv2025

The Milky Way Imaging Scroll Painting Survey: Data Release 1

We present the first data release (DR1) of the Milky Way Imaging Scroll Painting (MWISP) survey, a mapping in the J=(1-0) transition lines of 12CO, 13CO, and C18O toward the northern Galactic plane during 2011-2022. The MWISP survey was conducted using the PMO 13.7 m telescope at a spatial resolution of approximately 50" and a velocity resolution of 0.16 km/s at 115 GHz. DR1 fully covered 2310 square degrees within the Galactic longitude (l) and latitude (b) range of 9.75 deg =< l =< 229.75 deg and |b| =< 5.25 deg. The surveyed area was divided into cell units of 30'x30' for practical purposes and On-The-Fly (OTF) mapping was performed toward each target cell unit. The data were regridded into a regular 3D datacube in l-b-V_LSR with a pixel size of 30" in l-b axes and 0.16 km/s in theV_LSR axis. The median rms noise is 0.47 K, 0.25 K, and 0.25 K for 12CO, 13CO, and C18O, respectively. The equivalent 3 sigma sensitivity in 12CO luminosity is approximately 0.23 K km/s, making MWISP the most sensitive survey of its kind. In this paper, we describe the survey data, including the calibration, data cleaning, data mosaic processes, and the data products. The final mosaicked data cubes contain about 3.33x10^7 spectra (pixels) for each CO isotopologue line. Color composite images, made from the intensities of the isotopologue lines, and some concise descriptions are provided. We constructed a molecular cloud catalog based on the mosaicked 12CO data cube using the clustering algorithm DBSCAN, detecting 103,517 molecular clouds, 10,790 of which exhibit 13CO emission and 304 of which show C18O emission. Based on the histogram of voxel brightness temperature, we estimated a total 12CO flux of 7.69+/-0.38x10^7 K km/s arcmin^2, 82% of which is captured by the DBSCAN algorithm. The data, together with the cloud sample, provide unique information on molecular gas in the northern Milky Way.

astro-ph.GA↗

arXiv2025

Molecular Gas Distribution toward the Inner and Outer Galaxy Revealed by MWISP -- the Galactic Longitude 45°--60°and 120°--130°

Molecular clouds (MCs) are cradles of star and planet formation, thereby playing an important role in the evolution of galaxies. Based on the unbiased Milky Way Imaging Scroll Painting (MWISP) survey data of $^{12}$CO, $^{13}$CO, and C$^{18}$O (J=1--0) line emission in two regions toward the inner and outer Galaxy, i.e. the G50 ($44.75°\le l \le 60.25°$) and G120 ($119.75°\le l \le 130.25°$) regions, the distribution of molecular gas is studied. Both regions have Galactic latitudes of $|b| \le 5.25°$. A catalog containing 24724 MCs is constructed from the data. In our proximity, several molecular structures with large angular scales and small velocity dispersions are discovered, resembling curtains of mist. Beyond the nearby molecular gas, a clear aggregation of MCs along coherent structures in the Galactic plane is visible, sketching spiral arm structures. Nevertheless, the aggregation of MCs is also detected in the inter-arm region between the Perseus and Outer arms in the G50 region. The Galactic molecular disk in this inter-arm region is found to be thinner than that in the adjacent spiral arm region. In addition, the thickness of the Galactic molecular disk examined here is found to be correlated with the warp of it, indicating their homologous origins. The molecular disk has a typical thickness of ~220 pc in the inner Galaxy. Moreover, the dispersion of the MC systemic velocity decreases with increasing galactocentric radius, resulting in lower kinematic distance uncertainties at larger radii. However, the Perseus arm segment in the G120 region exhibits a relatively large cloud-to-cloud velocity dispersion and split components in its MC velocity distribution.

astro-ph.GA↗

arXiv2025

Cloud-to-cloud velocity dispersions across a Local arm segment

Using a large sample of 9617 molecular clouds (MCs) from the Milky Way Imaging Scroll Painting survey, we mainly measure one-dimensional cloud-to-cloud velocity dispersions across a 450 deg$^{2}$ segment of the Local arm in the Galactic second quadrant. We define the cloud-to-cloud velocity dispersion using two metrics: the standard deviation ($σ_{\rm bin}$) and flux-weighted root-mean-squared value ($σ_{\rm bin, w}$) of the centroid velocities of $^{12}$CO-detected MCs within spatial bins. The typical values of $σ_{\rm bin}$ and $σ_{\rm bin, w}$ are 7.5$\pm$0.5 km s$^{-1}$ and 6.2$\pm$0.5 km s$^{-1}$, respectively. After categorizing clouds by sizes into three types: Type $S$ (0.15 $-$ 1.2 pc), Type $M$ (1.2 $-$ 4.8 pc), and Type L ($\gtrsim$ 4.8 pc), we find that the spatial distribution of Type $S$ and $M$ MCs projected onto the Galactic longitude-latitude (l-b) plane is generally uniform. Additionally, the cloud-to-cloud velocity dispersion among Type $S$ clouds ($\sim$ 7.6 and 7.4 km s$^{-1}$ for $σ_{\rm bin}$ and $σ_{\rm bin, w}$, respectively) is systematically greater than that among Type $M$ clouds ($\sim$ 6.7 and 6.0 km s$^{-1}$ for $σ_{\rm bin}$ and $σ_{\rm bin, w}$, respectively), with differences of 0.9 $-$ 1.4 km s$^{-1}$. From these measurements, we estimate merger timescales between MCs to be approximately 0.3 to 0.9 Myr, which is shorter than their internal crossing timescales ($\sim$ 1 Myr for Type $S$, $\sim$ 2 Myr for Type $M$, and $\gtrsim$ 5 Myr for Type $L$). This disparity, particularly pronounced for larger Type $L$ clouds, suggests that MCs are dynamically transient structures, with their gas content changing due to frequent interactions with neighboring clouds.

astro-ph.GA↗

arXiv2025

Gas Transfer Between the Inner 3-kpc Disk and the Galactic Central Molecular Zone

We uncovered a more tilted molecular gas structure with highly negative velocities located near the dust lane. Our observations also show that the approaching gas flows from the overshoot process are captured by the bar gravitational and then flows towards the Galactic central molecular zone (CMZ) through the bar channel. The recycling gas from the overshoot effect, in conjunction with freshly accreted gas from the inner 3-kpc disk, accumulates significantly near R_GC~1/2R_bar and R_GC~2/3R_bar regions by adopting a bar length of ~3.2--3.4kpc. Importantly, within these regions, there are frequent collisions and substantial angular momentum exchanges between gas flows with different trajectories. In this scenario, the DISSIPATION processes arising from interactions between colliding flows, together with the varying torques induced by the nonaxisymmetric bar, effectively transfer the angular momentum of viscous gas outward, thereby driving the molecular gas to settle into the CMZ within ~3 orbital periods. A long-term gas inflow with an average rate of >1.1Msun/yr, coupled with intense transient accretion events that exceed the average rate by several times due to the overshoot effect, significantly regulates the gas distribution, physical properties, and dynamical evolution of the CMZ. These findings provide robust observational evidence for elucidating the intricate dynamics of molecular gas flows towards the CMZ. Our results show that gas dynamics has a significant impact on the secular evolution of both the Milky Way and the extragalactic gas-rich galaxies.

astro-ph.GA↗

arXiv2024

Revealing Gas Inflows towards the Galactic Central Molecular Zone

We study the gas inflows towards the Galactic Central Molecular Zone (CMZ) based on the gas morphological and kinematic features from the MWISP in the region of l=1.2 deg--19.0 deg and |b|<3.0 deg. We find that the near dust lane extends to l~15 deg, in which the end of the large-scale gas structure intersects with the 3 kpc-ring at a distance of ~5 kpc. Intriguingly, many filamentary MCs, together with the bow-like/ballistic-like clouds and continuous CO features with notable velocity gradient, are finely outlined along the long structure. These MCs also have relatively large velocity dispersions, indicating the shocked gas generated by local continuous accretion and thus the enhanced turbulence along the entire gas structure. We suggest that the ~3.1--3.6 kpc long CO structure originates from the accretion molecular gas driven by the Galactic bar. The gas near the bar end at the 3 kpc-ring becomes an important reservoir for the large-scale accreting flows inwards to the CMZ through the bar channel. The inclination angle of the bar is estimated to be 20--26 deg, while the pattern speed of the bar is 30--35 km/s. The total mass of the whole gas lane is about (0.9-1.7)x10^7 Msun according to the calculated X_CO=(0.6-1.4)x10^20 cm^-2 (Kkm/s)^-1 from the large-scale CO data and the complementary HI data. The mean gas inflow rate is about 0.8-1.4 Msun/yr, which seems to be comparable to the outflow's rate of the Galactic nuclear winds after applying the updated lower X-factor value above.

astro-ph.GA↗

arXiv2024

Bimodal orientation distribution and head-tail asymmetry of a sample of filamentary molecular clouds

The morphology of molecular clouds is crucial for understanding their origin and evolution. In this work, we investigate the morphology of the filamentary molecular clouds (filaments for short) using a portion of the $^{12}\text{CO} (J=1-0)$ data from the Milky Way Imaging Scroll Painting (MWISP) project. The data cover an area spanning $104.75^\circ <l< 150.25^\circ , \vert b\vert < 5.25^\circ$ in Galactic coordinates, with $V_\text{LSR}$ ranging from $-95$ to 25 $\text{km s}^{-1}$. Our primary focus is on the orientation and morphological asymmetry of the filaments. To achieve this, we apply several criteria on the data to create a sample of filaments with well-defined straight shape, and we use elliptical fitting to obtain the orientation of each filament, with an estimated error of $\sim1.6^\circ$ for the orientation. We find that the filament orientation with respect to the Galactic plane exhibits a bimodal distribution, a double-Gaussian fitting of which has two centres located at $-38.1^\circ $ and $42.0^\circ $, with 1$σ$ of the two Gaussian functions being $35.4^\circ$ and $27.4^\circ$. We do not find significant correlation between the orientation and other parameters, including the Galactic coordinates, radial velocity, velocity width, and physical scale. A considerable fraction of filaments ($\gtrsim 40$ per cent) display head-tail asymmetry, which suggests that mass concentration tends to occur at one end of the filaments.

astro-ph.GA↗

arXiv2024

Relative velocities between $^{13}$CO structures within $^{12}$CO Molecular clouds

Velocity fields of molecular clouds (MCs) can provide crucial information on the merger and split between clouds, as well as their internal kinematics and maintenance, energy injection and redistribution, even star formation within clouds. Using the CO spectral lines data from the Milky Way Imaging Scroll Painting (MWISP) survey, we measure the relative velocities along the line of sight ($Δ$V$_{\rm LOS}$) between $^{13}$CO structures within $^{12}$CO MCs. Emphasizing MCs with double and triple $^{13}$CO structures, we find that approximately 70$\%$ of $Δ$V$_{\rm LOS}$ values are less than $\sim$ 1 km s$^{-1}$, and roughly 10$\%$ of values exceed 2 km s$^{-1}$, with a maximum of $\sim$ 5 km s$^{-1}$. Additionally, we compare $Δ$V$_{\rm LOS}$ with the internal velocity dispersion of $^{13}$CO structures ($σ_{\rm ^{13}CO,in}$) and find that about 40$\%$ of samples in either double or triple regime display distinct velocity discontinuities, i.e. the relative velocities between $^{13}$CO structures are larger than the internal linewidths of $^{13}$CO structures. Among these 40$\%$ samples in the triple regime, 33$\%$ exhibit signatures of combinations through the two-body motion, whereas the remaining 7$\%$ show features of configurations through the multiple-body motion. The $Δ$V$_{\rm LOS}$ distributions for MCs with double and triple $^{13}$CO structures are similar, as well as their $Δ$V$_{\rm LOS}$/$σ_{\rm ^{13}CO,in}$ distributions. This suggests that relative motions of $^{13}$CO structures within MCs are random and independent of cloud complexities and scales.

astro-ph.GA↗

arXiv2023

Understanding the Kinetic Energy deposition within Molecular Clouds

According to the structures traced by $^{13}$CO spectral lines within the $^{12}$CO molecular clouds (MCs), we investigate the contributions of their internal gas motions and relative motions to the total velocity dispersions of $^{12}$CO MCs. Our samples of 2851 $^{12}$CO MCs harbor a total of 9556 individual $^{13}$CO structures, among which 1848 MCs ($\sim$ 65$\%$) have one individual $^{13}$CO structure and the other 1003 MCs ($\sim$ 35$\%$) have multiple $^{13}$CO structures. We find that the contribution of the relative motion between $^{13}$CO structures ($σ_{\rm ^{13}CO, re}$) is larger than that from their internal gas motion ($σ_{\rm ^{13}CO, in}$) in $\sim$ 62$\%$ of 1003 MCs in the `multiple' regime. In addition, we find the $σ_{\rm ^{13}CO, re}$ tends to increase with the total velocity dispersion($σ_{\rm ^{12}CO, tot}$) in our samples, especially for the MCs having multiple $^{13}$CO structures. This result provides a manifestation of the macro-turbulent within MCs, which gradually becomes the dominant way to store the kinetic energy along with the development of MC scales.

astro-ph.GA↗

arXiv2023

A Systematic Study of Associations between Supernova Remnants and Molecular Clouds

We universally search for evidence of kinematic and spatial correlation of supernova remnant (SNR) and molecular cloud (MC) associations for nearly all SNRs in the coverage of the MWISP CO survey, i.e. 149 SNRs, 170 SNR candidates, and 18 pure pulsar wind nebulae (PWNe) in 1 deg < l < 230 deg and -5.5 deg < b < 5.5 deg. Based on high-quality and unbiased 12CO/13CO/C18O (J = 1--0) survey data, we apply automatic algorithms to identify broad lines and spatial correlations for molecular gas in each SNR region. The 91% of SNR-MC associations detected previously are identified in this paper by CO line emission. Overall, there could be as high as 80% of SNRs associated with MCs. The proportion of SNRs associated with MCs is high within the Galactic longitude less than ~50 deg. Kinematic distances of all SNRs that are associated with MCs are estimated based on systemic velocities of associated MCs. The radius of SNRs associated with MCs follows a lognormal distribution, which peaks at ~8.1 pc. The progenitor initial mass of these SNRs follows a power-law distribution with an index of ~-2.3 that is consistent with the Salpeter index of -2.35. We find that SNR-MC associations are mainly distributed in a thin disk along the Galactic plane, while a small amount distributed in a thick disk. With the height of these SNRs from the Galactic plane below ~45 pc, the distribution of the average radius relative to the height of them is roughly flat, and the average radius increases with the height when above ~45 pc.

astro-ph.GA↗

arXiv2023

FAST reveals new evidence for M94 as a merger

We report the first high-sensitivity HI observation toward the spiral galaxy M94 with the Five-hundred-meter Aperture Spherical radio Telescope (FAST). From these observations, we discovered that M94 has a very extended HI disk, twice larger than that observed by THINGS, which is accompanied by an HI filament and seven HVCs (high velocity clouds) at different distances. The projected distances of these clouds and filament are less than 50 kpc from the galactic center. We measured a total integrated flux (including all clouds/filament) of 127.3 ($\pm$1) Jy km s$^{-1}$, corresponding to a H I mass of (6.51$\pm$0.06)$\times$10$^{8}$M$_{\odot}$, which is 63.0% more than that observed by THINGS. By comparing numerical simulations with the HI maps and the optical morphology of M94, we suggest that M94 is likely a remnant of a major merger of two galaxies, and the HVCs and HI filament could be the tidal features originated from the first collision of the merger happened about 5 Gyr ago. Furthermore, we found a seemingly isolated HI cloud at a projection distance of 109 kpc without any optical counterpart detected. We discussed the possibilities of the origin of this cloud, such as dark dwarf galaxy and RELHIC (REionization-Limited HI Cloud). Our results demonstrate that high-sensitivity and wide-field HI imaging is important in revealing the diffuse cold gas structures and tidal debris which is crucial to understanding the dynamical evolution of galaxies.

astro-ph.GA↗

arXiv2023

FAST discovery of long tidal tails in NGC 4490/85

We report the discovery of a 100 kpc HI tail in the merging galaxy pair NGC 4490/85 detected by the Five-Hundred-meter Aperture Spherical radio Telescope (FAST). The tidal tails extended in both the south and north directions, and they are much longer than that reported previously based on the VLA interferometric maps. The NGC 4490/85 is surrounded by a large gas envelope, and a starburst low metallicity dwarf galaxy MAPS 1231+42 is found to be connected with the gas envelope, indicating that galaxy interaction trigged the intense star formation in it. Based on the fact that the metallicity in MAPS 1231+42 is one order of magnitude lower than that in the two disks of NGC 4490 and NGC 4485, we speculate that the gas near this galaxy should be primordial and could be due to gas inflow from the circum-galactic medium (CGM). We also found a collimated gas component pointing at a nearby dwarf galaxy KK 149, suggesting that this galaxy might also be interacting with the NGC 4490 pair. We discuss the possible origin of the long tidal tails and the extended gas envelope in this merging system based on the new data from FAST.

astro-ph.GA↗